1. Field of the Invention
The present invention is in the field of manufacturing glass from the gas phase wherein glassforming reaction gases are introduced into a reaction zone wherein a chemical reaction is induced between said combined reaction gases to produce at least one glass composition being deposited from said reaction zone on a solid surface in a deposition zone as a soot or powder.
2. Description of the Prior Art
Fiber optical waveguides for optical communications are presently almost exclusively produced from preforms which are themselves produced by the glassforming reaction from the gas phase and deposition on a solid surface. The glasses deposited by this method possess the quality required for the optically effective core and cladding of an optical fiber. In any such optical fiber, the refractive index of the core is greater than the refractive index of fiber cladding on the core.
There have been three types of processes applied for the manufacture of preforms which are identified as follows:
(a) The MCVD method (modified chemical vapor deposition method as described in Siemens Forschungs--und Entwicklungsberichte, Vol. 5 (1976), No. 3, pages 171-175) where the deposition occurs in the inside of a glass tube; PA0 (b) the so-called OVD method (outside vapor deposition method) wherein an outside deposition is performed on the generated surface of an auxiliary rod; and PA0 (c) the so-called VAD method (vapor axial deposition method) wherein an outside deposition occurs on the end face of a rod.
All of the three processes mentioned above have in common that vaporous SiCl.sub.4 is mixed with one or more vaporous dopants, for example GeCl.sub.4 and POCl.sub.3, for raising or lowering the refractive index and are converted into oxides in a hot reaction zone by oxidizing with oxygen. They are deposited on a relatively cold surface in powdery or sooty form. In a succeeding step, the powdery material is fused or sintered into glass. The final product is a glass rod, which can be used as a preform from which an optical fiber can be drawn.
A more detailed description of the preparation of low loss, optical fused silica fiber appears as example for all methods in the aforementioned article describing the MCVD method. This article is incorporated herein by reference. As disclosed in this article, volatile chemical compounds such as the chlorides of silicon, germanium, titanium, tin and phosphorus are used as starting materials for the chemical vapor deposition of glass. These compounds are transported by a flow of oxygen into a commercial fused silica tube where they are oxidized at high temperatures. The oxidation of SiCl.sub.4 starts at about 900.degree. C. to produce mostly low molecular weight oxychlorides which can then be oxidized at or above 1400.degree. C. to form SiO.sub.2. At these temperatures, the chemical equilibrium of the reaction favors formation of non-volatile oxides, while at about 1700.degree. C. the volatile compounds GeO.sub.2 and BOCl are favored.
The oxides are first deposited as a layer of soot or powder on the inner surface of the tube and then turned into glass by a fusion process. This allows realization of large mole fractions of the chlorides in the gas mixture and consequently high soot or powder deposition rates.
The refractive index of pure fused silica can be raised by the addition of oxides such as GeO.sub.2, P.sub.2 O.sub.5, TiO.sub.2, SnO.sub.2 and lowered by the addition of B.sub.2 O.sub.3. It is common to deposit SiO.sub.2 --B.sub.2 O.sub.3 layers successively on the inner surface of a fused silica tube followed by SiO.sub.2 --GeO.sub.2 layers with a higher refractive index to build up a material having the proper optical properties.
The methods described produce superpure and thus low attenuating fiber optical waveguides. However, the production rate is still relatively low in general. Furthermore, a large part of the vaporous material is lost as exhaust gas.